Answer:
[tex]\boxed{\text{Replacing the cube of manganese(IV) oxide with its powder}}[/tex]
Explanation:
The MnOā is acting as a catalyst: it takes part in the reaction but can be recovered at the end.
A. Using a 2 g cube of MnOā
The surface area of the catalyst would double, so the rate would probably double. Technically, this option is correct, but I don't think it is the one the questioner intended.
B. Using 50 cm³ of HāOā
No effect. The rate depends on how fast the HāOā molecules can reach the surface of the catalyst, and the surface area hasn't changed.
C. Using powdered MnOā
This would increase the rate enormously, because the powdered catalyst has a much greater surface.
D. Removing MnOā
The rate would decrease to near-zero, because there is no catalyst.